The two armies will try to a truce to end the bloodshed of the long war. which term best completes the sentence? a exclude d elaborate

You didn't include all available options which are exclude, negotiate, concede, and elaborate. In this case, the correct answer is negotiate.
The two armies will try to talk and find a solution which is best for both parties in order to bring the long war to a finish. This is the meaning of the word negotiate, which is the correct answer here. 
To exclude means to omit, to concede means to give up, and to elaborate means to explain in further detail.